The question asks about the specific graph drawn between the dry density of soil and the water content at the time of compaction. This graph is fundamental in soil mechanics and civil engineering, particularly for understanding the behavior of soil under compaction efforts.

Dry Density and Water Content Graph

When soil is compacted, its properties change depending on the amount of water present. The relationship between the dry density ($\rho_d$) of the soil and its water content ($w$) during compaction is crucial. A series of compaction tests are performed on a soil sample, where the same compaction effort is applied but at varying water contents. For each water content, the resulting bulk density and water content are measured, from which the dry density is calculated using the formula:

$$ \rho_d = \frac{\rho_{bulk}}{1 + w} $$

Where $\rho_d$ is the dry density, $\rho_{bulk}$ is the bulk density, and $w$ is the water content (as a decimal).

Moisture Density Curve Explanation

The graph plotted using these calculated dry density values against their corresponding water content values is known as the Moisture density curve. This curve typically rises to a peak and then falls, forming a bell-like shape. The peak of this moisture density curve represents two critical parameters:

• Maximum Dry Density (MDD): This is the highest dry density that can be achieved for a given compaction effort.
• Optimum Moisture Content (OMC): This is the water content at which the maximum dry density is achieved.

The Moisture density curve, also commonly referred to as the compaction curve or Proctor curve (after R.R. Proctor who developed the standard compaction test), is extensively used in the field for quality control during earthwork construction, such as building road embankments, dams, and foundations. It helps engineers determine the optimal conditions to achieve the desired soil density for stability and strength.

Compaction Curve Significance

Understanding the compaction curve is vital for several reasons:

• It helps in selecting the appropriate water content for field compaction to achieve maximum density and strength.
• It provides insight into the soil's behavior under different moisture conditions during compaction.
• It is a standard laboratory test (Proctor test) used to establish compaction specifications for construction projects.
• Achieving the optimum moisture content ensures the most efficient use of compaction energy to densify the soil.

Soil Curve Comparisons

Let's briefly consider why the other options are not correct in the context of the graph drawn between dry density and water content at the time of compaction:

• Compression curve: This typically relates to the change in void ratio or strain with applied pressure, commonly associated with consolidation tests, not compaction.
• Settlement Curve: This graph shows the decrease in volume (settlement) over time due to applied load, primarily studied in consolidation analysis for structures.
• Porosity curve: While porosity is related to soil density, a "porosity curve" as a standard graph drawn directly against water content at compaction isn't the primary or most common term for this specific relationship. The dry density curve inherently shows the effect on void ratio/porosity, but the graph itself is named for density and moisture.

Therefore, based on the definition and application in soil mechanics, the graph representing the relationship between dry density and water content during compaction is correctly known as the Moisture density curve.